Method and apparatus for bonding thermoplastic materials

ABSTRACT

A method and apparatus is disclosed for bonding two pieces of thermoplastic material to one another. The method includes the steps of heating the edges of the plastic materials to be joined to at least their fusing temperatures and then forcing the heated edge portions against one another to thereby form a bonded junction. A bead is formed along at least one edge of the junction of the plastic materials as a result of the pressure of the two plastic materials bearing against one another. The welded junction is heated, optionally at elevated pressure, to at least its fusion temperature and is then rapidly cooled. The resulting weld has a high impact and dielectric strength and has a smooth overall appearance.

This is a continuation of application Ser. No. 63,926 filed Aug. 6,1979, now abandoned, which is a continuation of application Ser. No.862,559, filed Dec. 20, 1977, which is abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an improved method and apparatus for bondingthermoplastic materials to one another.

Techniques for bonding thermoplastic materials to one another have beenknown for quite some time. Examples of such bonding techniques aredescribed in Welding of Plastics, Neumann and Bockhoff, ReinholdPublishing Co., 1959, and include hot plate and friction welding. Byeither of these techniques, the edges of the plastic materials to bebonded are heated to bring the plastic at the edges to its fusiontemperature. As soon as the edges are sufficiently heat-softened, theyare quickly joined together under pressure until the melted or softenededges have cooled sufficiently to form a strong joint. During thewelding operation, the pressure between the two softened edges of thethermoplastic materials should be sufficient to force out air bubblesand to bring the entire edge surfaces into intimate contact. Theresulting pressure on the softened edges as they are joined togetherresults in the formation of a rounded bead along the junction of the twothermoplastic materials. In the past after the bonded or welded edgescooled, the rounded bead was removed by sanding in an area about thejuncture of the bonded edges or by cutting away the bead. This wasfollowed by a polishing step.

In many applications, however, the integrity, reliability and durabilityof the weld or bond is of critical importance. As one example, whenthermoplastic pipes are bonded to one another by means of a hot plateweld, it is very important that the weld have the required strength anddurability in order to serve the purpose of conveying fluids undervarying temperatures and pressures in an environment which may besubject to substantial vibrations. As a second example, some batteryjars are formed by hot plate welding techniques. These battery jarscontain a liquid electrolyte and support a series of heavy electrodes.When placed in situ, the battery jars are subjected to vibration andoccasional shock impulse forces, and accordingly, the welds must be ofsubstantial strength and durability to remain functional over a longperiod of time.

In order to test the integrity and reliability of these welds a numberof techniques have been developed. One method is to establish a veryhigh electromagnetic field across the weld to determine whetherdielectric breakdown occurs. If there are minute pores and/or cracks inthe weld, the dielectric strength of the weld will be reduced and uponestablishing the electromagnetic field across the weld, a spark will begenerated.

Another technique for testing the integrity and reliability of welds isto generate a mechanical impulse force against the weld to determine itsresistance to fracture. In the battery jar industry this is accomplishedby dropping a weighted dart from a preset distance onto the weld togenerate a very high point pressure differential across the weld. Ofcourse, other impact techniques can be used depending upon the designrequirements of the finished product. These techniques for measuring thereliability and strength of welds have proven useful in manyapplications where the integrity of a weld joint is of criticalimportance.

Using these and other known testing techniques, it has been found thatthe formation of hot plate welds by the simple heating of the edges ofthe thermoplastic materials to be joined and then forcing the edgesagainst one another to form the weld results in decreased tensilestrength of the material at the weld junction; that is the tensilestrength of the material at the weld junction can be 85 percent of thetensile strength of parent material and lower. In addition, thedielectric test failure rate resulting from generating a largeelectromagnetic field across the weld increases as much as 100 timesover the dielectric test failure rate of the parent material. Further,the impact strength of such welds when tested by dropping a dart ontothe weld was found to be reduced substantially over that of the parentmaterial and in addition varied substantially at different points alongthe welds and from one weld to the next to thereby reduce the overallreliability of the weld. Further, the bending strength, particularly theflexural deflection, of the weld about the axis of the weld was found tobe reduced substantially.

It is therefore an object of this invention to provide an improvedmethod of bonding thermoplastic materials to one another to improve thestrength and reliability of the bond.

It is another object of this invention to provide an improved apparatusfor bonding thermoplastic materials to one another.

SHORT STATEMENT OF THE INVENTION

Accordingly, this invention relates to an improved method and apparatusfor bonding thermoplastic materials to one another. The method comprisesthe steps of heating the edges of the thermoplastic materials to atleast their fusion temperatures. The heated edges are then forcedagainst one another to thereby form a welded junction. A bead is formedas a result of the pressure of the two thermoplastic materials bearingagainst one another with the bead being formed along the junction of theweld. Then the weld junction (1) is heated to at least about its fusiontemperature, optionally at elevated pressure, and then (2) is rapidlycooled to improve the physical properties of impact strength, dielectricstrength and flexural deflection of the plastic material proximate theweld junction.

The apparatus includes known equipment for heating and joining the edgesof the thermoplastic materials to thereby form a welded junction. Theimproved apparatus of the present invention includes a strip of materialwhich can be heated and cooled relatively rapidly. The strip which ispreferably the shape of the weld junction is supported by an insulatingmaterial which has a grooved network throughout the surface thereofwhich supports the strip. The strip may be heated, for example, by anelectric current and is cooled by drawing air from the area surroundingthe strip through the groove network and out through a vacuum pump.

In operation, after forming the welded junction, the strip of materialis forced against the welded junction that has been formed during thewelding step and is heated to approximately the fusion temperature ofthe plastic material. The heated weld junction area and the strip arethen rapidly cooled by drawing ambient air past the junction area andthe strip through the groove network. When the plastic material hascooled sufficiently, the strip is removed from the plastic material.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome fully apparent from the following detailed description of thepreferred embodiments, the appended claims and the accompanying drawingsin which:

FIG. 1 is a simplied illustration of a welded joint having a roundedbead formed on each side of the weld;

FIG. 2 is a simplified perspective view of one embodiment of theapparatus for forming an improved hot plate weld;

FIG. 3 is a perspective view of the preferred embodiment of theapparatus for forming an improved hot plate weld;

FIG. 4 is a cutaway side section view, illustrated in an enlarged scale,of the apparatus of FIG. 3;

FIG. 5 is an enlarged section view of a weld made in accordance with theprocess of the present invention;

FIG. 6 is a cross-section view of a simplified apparatus using theembodiment of FIG. 3;

FIG. 7 represents two end sections which can be welded together to forma battery jar;

FIG. 8 is a side elevation view of the battery jar formed by welding thetwo end sections depicted in FIG. 7; and

FIG. 9 is a plan view of the battery jar of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 there is a cross-sectional view of a weld joint formed byheating the respective edges 10 and 12 of two pieces of thermoplasticmaterial. After the respective edges have been heated to their fusiontemperature or until they become plastic, the edges are forced againstone another to form a welded junction. The pressure on the moltenplastic edges of the thermoplastic material resulting from forcing theedges against one another creates a rounded bead 11 on each side of theweld. The dotted lines 13 and 13' near each edge 10 and 12 illustrate,in a simplified manner, that portion of the thermoplastic material whichwas reheated during the welding process. Weld failure resulting from theaforementioned dart impact test procedure, frequently occurs between andalong the respective boundary lines 15 between the reheated portions 13and the non-heated portions 17 of the thermoplastic material.

Weld failure, as measured by the dart impact test procedure, indicatesthat the thermoplastic material at the welded junction is more brittleand less ductile, than the parent material. While decrease in tensilestrength of the material at the welded junction has been noted,brittleness and loss of ductility of the material at the weld junction,compared to the parent material, are more serious side effects of thewelding process. Moreover, the material at the junction is characterizedby a much higher rate of dielectric test failure, according todielectric requirements of the industry, compared to the parentmaterial.

Various reasons for the variations in weld impact strength at the weldedjunctions were proposed: (1) The molecular weight distribution of thethermoplastic material might influence weld impact strength and mightaccount for the variations; (2) the material was becoming oxidizedduring welding and was, therefore, more brittle; and (3) the crystallinestructure of the material in and adjacent to the weld line was coarseneddue to the welding heat. However, it could not be established that anyone or combination of these reasons resulted in the decrease of impactstrength of the thermoplastic material at the weld junction.

In accordance with the invention, it was discovered that impactstrengths of the material at the welded junction could be increased andthat dielectric test failure of the material at the welded junctioncould be substantially eliminated, (1) by heating the material at theweld junction to a temperature at least about the fusion temperature ofthe thermoplastic material, optionally at elevated pressure, and (2) byquickly quenching the heated junction to a temperature at least belowthe fusion temperature. As stated above, when welding two pieces ofthermoplastic material, a bead occurs along at least one side of thewelded junction. In accordance with the process of the invention, thebead can be removed prior to the steps of heating and quenching, butpreferably it is not removed.

The exact temperature of heating will depend on the exact thermoplasticmaterials which have been welded, and, for instance, can be as low as300° F. for branched polyethylene and can be up to 900° F. when thethermoplastic is high density polyethylene thermoplastic. That is, theexact temperature of heating will depend on the fusion temperature ofthe thermoplastic material, i.e., that temperature at which it becomesmolten. As a practical guideline, the exact temperature of heating canbe determined for a specific thermoplastic by selecting that temperatureat which sufficient fusion occurs within a period of time up to about 25seconds.

Pressure is applied to the weld junction during the step of heating orafter the step of heating. The pressure must be sufficient to cause thematerial at the weld junction to become substantially flat. In practice,the pressure can vary widely depending on the apparatus and temperaturesused and can range from 20 to 90 lb./square inch. If the bead formed atthe weld junction is not removed, the pressure must be sufficient tomash the bead against the welded junction until it is substantiallyflat. Preferably, in the preferred embodiment pressure is applied duringthe heating step and the combined effect of the conditions of heat andpressure is sufficient to mash the bead until it is substantially flat.

After the pressure treatment, the heated, welded junction is immediatelyquenched. Quenching comprises rapidly cooling the heated, weldedjunction to a temperature at least below the fusion point of thethermoplastic material and preferably to a temperature at which thethermoplastic lacks adhesive properties. The quenching step isundertaken to resolidify the material at the welded junction.

Quenching must immediately follow pressure or heat-pressure treatment ofthe welded junction. That is, quenching in accordance with the inventiondoes not include allowing the pressure or heat-pressure treated weldedjunction to cool at ambient conditions. Surprisingly, quenching afterthe steps of fusing the two edges of thermoplastic and joining thoseedges under pressure, i. e., immediately after formation of the weldedjunction, does in practice result in improved properties of the weldjunction, with respect to dielectric properties, impact strength andflexural deflection about the axis of the weld. Quenching may beundertaken, for instance, by immersing the treated junction into water.

In order to overcome the problem of decreased tensile strength, lowimpact strength, high dielectric test failure rates, and the problemsresulting from having a rounded bead extending along the longitudinallength of the weld, an apparatus has been developed which in itssimplest form is illustrated in FIG. 2. In FIG. 2, there is illustratedan insulating strip 19 which, in the preferred embodiment, is a ceramicmaterial or a high temperature plastic such as Torlon, a polyamide. Astrip 21, preferably of Titanium Alloy 6AL4B having a thickness of 0.30millimeter and a width of 25 millimeters, is positioned over theinsulating strip 19. As will be seen, the insulating strip 19 serves thedual function of an electrical and heat insulator and as a mechanism forrapidly cooling, among other things, the strip 21.

In the annealed condition, the strip 21 has an electrical resistivity ofapproximately 180 micro-ohms-centimeter, excellent corrosion resistance,and a tensile yield strength of 130,000 lbs per square inch at roomtemperature. The high strength of the alloy is useful in resisting thelocal pressure forces generated when first contacting the rounded weldbead. The high strength is also useful due to the forces imposed uponthe Titanium strip when subjected to high temperatures. As an example,when the Titanium strip is heated to 450°-500° F. it increases in lengthdue to thermal expansion. On the other hand, the plastic materialoutside of the weld zone, i.e., the area 13, is substantially able tokeep the surface area of the bead about the heated strip 21 fromexpanding or contracting along the weld junction during the process ofmashing the bead and cooling the resulting mashed bead.

Consequently, the longitudinal expansion and contraction of the heatedstrip 21 results in a shear stress between the strip 21 and the weldbead material. This could lead to flaws in the surface of the plasticmaterial and possible warpage of the plastic device being formed by thewelding step. Accordingly, the Titanium Alloy strip 21 is placed under alongitudinal tensile strain at room temperature which is slightlygreater than the maximum thermal strain which occurs during heating andcooling. The strain is maintained constant during the heating of thestrip 21 by techniques known in the art. For instance, this strain canbe effected by screws 55 and 56 in FIG. 6. In this manner each pointalong the alloy strip 21 remains in substantially the same location withrespect to the bead during heating and cooling, and accordingly thelength of the heated section of the strip 21 remains substantiallyconstant. Since a room temperature stress of approximately 35,000 poundsper square inch is necessary to provide the necessary strain on thestrip, which stress is reduced substantially under high temperature, thestrength of the strip 21 must be quite high. It can be clearly seen thatthe 130,000 lbs/square inch tensile yield strength is more than adequatefor the stress levels induced into the strip 21.

As illustrated in FIG. 1, the heated strip 21 together with itsinsulating support 19 is pressed against the bead 11 to cause it to fuseand become plastic. The bead is pressed and flattened against the weldarea. During this operation, the fused thermoplastic material willadhere to the strip 21. After the strip is cooled below the meltingpoint of the thermoplastic material, the adhesion of the strip to thethermoplastic material ceases and the strip can be removed from thematerial.

It has been discovered that the welded joint which has been heated inaccordance with the invention must be rapidly quenched in order torealize the advantage of improved tensile strengths, impact strengthsand dielectric properties of the material at the welded junction.Accordingly, a plurality of holes 23 are formed in the strip 21, each ofthe holes being in communication with one another through a trough 25formed in the insulating support 19. In one embodiment cool air is blownthrough the trough 25 and out through the holes 23 about the heatedthermoplastic material as illustrated by the arrows in FIG. 2. Thisquickly cools the thermoplastic material and the strip to therebyprovide the desired crystalline structure, i.e., the mashedthermoplastic material illustrates a smooth, closed surface having avery low dielectric failure rate. The remolded weld bead forms anadditional flat layer of material which becomes laminated to the parentmaterial. Thus, the possibility of a minute flaw in the weld causingundesirable leakage is substantially reduced.

FIGS. 3 and 4 represent the preferred embodiment of the apparatus of thepresent invention. As illustrated, an insulating strip 29, formed, forinstance, of ceramic or high temperature resistant plastic, has a groove35 formed through the center thereof with a plurality of transversegrooves 33 of relatively small size being formed along the length of theinsulating strip 29. Positioned over the insulating strip 29 is a heaterband or strip 31 which preferably is formed of Titanium Alloy 6AL4Vhaving a thickness of 0.30 mm and a width of 25 mm. This strip, asaforementioned in connection with discussion of the embodiment of FIG.2, initially is strained at room temperature to a level greater than themaximum strain due to heat in order to maintain the position of thestrip in the same location with respect to the thermoplastic bead duringthe hot mashing operation.

In the embodiment of FIG. 3, ambient air is sucked in through thegrooves 33 by means of a vacuum pump (not shown) which establishes areduced air pressure level of 0.2 atmospheres. By sucking cool ambientair in through the grooves 33, a more uniform distribution of air aboutthe strip 31 and the mashed bead is created, and hence a more uniformcooling of the strip 31 and the mashed thermoplastic material isachieved.

The groove 35 should have a relatively small width in order to providesupport for the strip 31, and accordingly the groove must be deep inorder to channel the sucked in air from each of the grooves 33 to thevacuum pump. In addition the grooves 33 should be sufficiently wide topresent a large cooling area to the strip 31 but should not be so widethat the strip 31 is not given adequate support.

In the preferred embodiment the grooves 33 are 1.7 mm wide and only 0.17mm deep, with each groove separated by a 0.5 mm land. This groovestructure is designed to keep the bending stress in the strip 31 smalland at the same time to present a relatively large area of the strip tothe cooling air. At the same time, during the heating cycle, the groovesact as insulators preventing a large heat transfer to the insulator 29.The central trough 25 is deep and narrow so that it presents very littlesurface area to the strip 31 which might induce transverse bendingstresses while at the same time has a sufficiently large crosssectionalarea to conduct the air from grooves 33 to the vacuum pump.

Using the embodiment of FIG. 2 or FIGS. 3 and 4, when electricity isconducted through the strip 21 or 31 it becomes sufficiently hot tobring the bead 11 illustrated in FIG. 1 to about or above its fusion ormelting temperature. The support 20 for the insulator 19 or 29 and thestrip 21 or 31 forces the heated strip against the bead to mash the beadagainst the weld area until substantially flat. The reheated beadmaterial then becomes bonded to the plastic of the weld area asillustrated in FIG. 5 to form an improved weld joint. The joint of FIG.5 is shown out of scale in order to clearly illustrate how the mashedbead forms a thin extra layer of bonded plastic material at the weldjunctions.

Turn now to FIG. 6 which is a simplified cross-sectional view of anapparatus for making battery jars which use the embodiment of FIG. 3.The strip 31 is disposed over insulator material 29 which, in turn issupported on a steel frame 60 which defines an enclosed space S. Thespace S is in communication with vacuum pump P and opens to trough 35,which in turn communicates with grooves 33. When the vacuum pump isactuated, it draws air under reduced pressure over the strip 31 and thewelded junction area and acts to cool both. A copper coating 57 (shownin exaggerated form for clarity) is disposed on strip 31 on those areasof strip 31 which do not contact the plastic material to prevent thestrip from overheating in these areas.

The strip 31 must be maintained under strain as indicated during theaforementioned discussion of FIG. 2. Screws 55 and 56 schematicallydepict one set of means for effecting this strain; but obviously thereare many recognized equivalents which can be used instead. On turningthe screw 56, an end of strip 31 is wound, thereby to provide thenecessary strain on strip 31. As shown in FIG. 6, the insulator material29 on which strip 31 is supported is disposed on a flat surface.However, the surface which supports the insulator material need not beflat but may have a surface which conforms to the surface ofthermoplastic workpiece at the welded junction. Thus, if twothermoplastic pipes are welded together the surface will be annular orcylindrical conformation. A piston and cylinder arrangement actuates theframework 60 to provide contact between the weld junction 41.

A second apparatus 58 is illustrated in schematic form on the oppositeside of the junction 41 of the plastic material 14 and serves to heatand mash the bead formed at the other side of the junction 41.

EXAMPLE

In operation, the embodiment of FIGS. 3 and 4 using the apparatus ofFIG. 6 was applied to making a battery jar of a propylene-ethylenecopolymer blend, of the type represented by FIGS. 7-9.

The elongated battery jar of FIGS. 8 and 9 8 comprises two end members37 and 39, as illustrated in FIG. 7, each of which is open at the topthereof and has an elongated open side and three elongated closed sides.The distance from the surface defining the elongated open side of eachmember to its opposed closed side is at least several times smaller thanthe distance from the top to the bottom thereof. Typically, the distancebetween the open side and the opposed side is 1/4 to 1/10 the distancefrom the top to the bottom of members 37 and 39. Each end member 37 and39 has a wall thickness which is substantially the same from the top tothe bottom thereof; i.e., there is no taper or draft from top to bottomand each of the end members 37 and 39 is a mirror image of the other.The end members 37 and 39 are heat welded at the respective elongatedends to form the battery jar--illustrated in FIGS. 8 and 9.

The manner and method of making the battery jar end members 37 and 39 isdisclosed in copending U.S. patent application Ser. No. 648,738 of JamesS. Hardigg filed Jan. 13, 1976 now U.S. Pat. No. 4,118,265, the subjectmatter of which is incorporated herein by reference thereto.

The primary requirements for a battery jar are that it be resistant tothe battery acid, have no leaks, have substantial dimensional accuracy,be resistant to shrinkage when the battery is overheated, have highimpact strength to withstand accidents during battery manufacture anduse, have uniform width and length from top to bottom, that is, nodraft, have straight sides which are not bowed out or in, and have acapacity to bend and/or deform during handling in order to prevent thefracture thereof.

As aforementioned as the respective edges of end section members 37 and39, illustrated in FIG. 7, are heated to the fusion temperature and thenjoined to one another to form a weld, the fused plastic material formsbeads 11 on the inside and outside of the jar of the type illustrated inFIG. 1 and FIGS. 8 and 9. After the welded junction including the beadshas cooled, the insulator 29 and band 21 illustrated in FIG. 3 arepositioned along both the inside and outside weld area against the beadsformed during the initial welding step by using an apparatus of the typeillustrated in FIG. 6.

The Titanium Alloy strip 21 is then heated over a time interval rangingfrom 2.5 seconds to over 20 seconds while in pressing engagement withthe beads. The beads thereby fuse and become flattened against theheated weld area 13 illustrated in FIG. 1 to thereby form a flattenedweld joint as illustrated in FIG. 5. The strip 21 and the mashed beadare then cooled by drawing air at room temperature through the groovesand the trough formed in the insulating strip 29. After the mashed beadhas cooled sufficiently to no longer adhere to the strip 21, the stripand insulating support were removed to form the final welded batteryjar.

It has been discovered that when longer heating and cooling times areused, the dart-impact strength of the welded joints increases. However,it has also been discovered that as longer heating cycle times are used,the battery jars warp, particularly at the upper end adjacent to theopen end of the jar, i.e., the jars bow inward or outward to anunacceptable extent. The warpage resulting from long heating timesapparently is due to the shrinkage which occurs in the plastic materialafter heating it to the melting point. Thus, the material in the areaover which the bead is mashed is brought to or near the melting pointthereof and accordingly shrinks during cooling, whereas the surroundingmaterial which has not been reheated does not shrink.

One technique for overcoming the warpage problem is to preheat thewelded battery jars to 180°-200° F. prior to the mashing process. Thiscauses the entire jar to shrink somewhat upon cooling, and accordinglythe differential in shrinkage between the material adjacent to the weldand the remainder of the battery jar is substantially reduced. Thistechnique, however, is not desirable on a production line basis sincethe lengthened cooling cycle required with preheated jars substantiallyincreases the total manufacturing time of the battery jars. It hastherefore been discovered that by using a very short heating time in therange of 3 to 4 seconds and heating the Titanium Alloy strips to ahigher temperature, an improved weld having high dart-impact strengthwith substantially no warpage results. The extent of warpage was furtherreduced by utilizing a technique of drawing relatively cool ambient airin under the strip 31 which has the effect of cooling the battery jarmaterial adjacent to the strips. This results in a narrower zone ofheated plastic material subject to shrinkage which in turn reduces thedistortion in the walls of the battery jar due to shrinkage. Thus, byusing a relatively short heating time cycle and drawing air in from thearea surrounding the heated plastic material, the overall cycle time fortreating the welded junction falls below 30 seconds.

While the present invention has been disclosed in connection with thepreferred embodiments thereof, it should be understood that there may beother modifications to the invention which fall within the spirit andscope thereof as defined by the appended claims.

What is claimed is:
 1. A method of bonding two pieces of thermoplasticmaterial to one another to form an article, consisting essentially ofthe steps of:heating at least one edge of each of said two pieces to atleast about the fusion temperature of said thermoplastic material; thenbutt joining said heated edges to one another under pressure to therebyform a welded junction, said pressure causing a bead of plastic materialto be formed on at least one side of said welded junction; and improvingthe physical and dielectric strength of the material of said weldedjunction by subjecting the welded junction to a reheat treatment tocause the material to become molten, and quenching the molten materialto reduce its temperature to a temperature at least below said fusiontemperature.
 2. The method according to claim 1 wherein pressure isapplied normal to the welded junction during said reheating step inorder to cause the material at the welded junction to becomesubstantially flat.
 3. The method according to claim 1 wherein said beadis reheated and pressed against said welded junction until substantiallyflat.
 4. The method according to claim 1 wherein said bead is removedfrom said junction prior to said step of reheating.
 5. A method ofmaking an elongated thermoplastic battery jar having a first end memberwhich is open at the top and closed at the bottom thereof and which hasan elongated open side and three elongated closed sides, the distancefrom the surface defining the open side to its opposed closed side beingat least several times smaller than the distance from the top to thebottom thereof, and a second open sided elongated end member being amirror image of said first end member, the method consisting essentiallyof the steps of:heating the edge which defines the elongated open sideof said first open sided end member to at least its fusion temperature;heating the edge which defines the elongated open side of said secondopen sided end member to at least its fusion temperature; butt joiningsaid end members at the respective heated edges thereof to form a weldedjunction, a bead of plastic material being formed along the axis of saidwelded junction; and improving the physical and dielectric strength ofthe material at the welded junction by reheating the material at thewelded junction to a temperature at least about the fusion temperatureof the thermoplastic material, and rapidly cooling the heated materialat the welded junction to a temperature below the fusion temperaturethereof to thereby form a battery jar.
 6. The method of claim 5 whereinsaid reheating step is undertaken at an elevated pressure whereby saidbead is mashed against said welded junction.
 7. A method of making anarticle of manufacture which is formed of at least two thermoplasticmembers which are welded together to form said article, wherein an edgeof a first of said members is welded to an edge of a second of saidmembers, and wherein the edge of said first member is a mirror imageconfiguration of said edge of said second member, the method consistingessentially of the steps of:heating the edge of said first member to atleast its fusion temperature; heating the edge of said second member toat least its fusion temperature; butt joining said heated edges of saidmembers to form a welded junction, a bead of plastic material beingformed at said welded junction; and improving the physical anddielectric strength of said material at the welded junction by reheatingsaid bead to at least its fusion temperature, and rapidly cooling saidreheated material at the welded junction to a temperature below thefusion temperature thereof to thereby form said article of manufacture.8. The method of claim 7 wherein said reheating step is undertaken atelevated pressure whereby said bead is mashed against said weldedjunction until substantially flat.
 9. The method of claim 7 wherein saidarticle of manufacture is a thermoplastic container.
 10. The method ofclaim 7 wherein said article is a thermoplastic pipe conduit.
 11. Amethod of bonding two pieces of thermoplastic material to one another toprovide improved strength at the junction of said pieces, said pieceseach having at least one edge surface which mates with an edge of saidother piece, the method consisting essentially of the stepsof:separately heating said at least one edge of each of said first andsecond pieces to about their fusion temperatures; joining said heatededges to one another in an non-overlapping relationship under pressureto thereby form a welded junction, said pressure causing a bead ofplastic material to be formed on at least one side of said weldedjunction; and improving the physical and dielectric strength of saidmaterial at the welded junction by reheating said welded junction to atleast about its fusion temperature to cause the material at the weldedjunction to become molten, and rapidly cooling said molten material to atemperature at least below said fusion temperature.
 12. The methodaccording to claim 11 wherein said pressure is applied normal to thewelded junction during said reheating step in order to cause thematerial at the welded junction to become substantially flat.
 13. Themethod according to claim 11 wherein said bead is pressed against saidwelded junction until substantially flat during said reheating step. 14.The method according to claim 11 wherein said bead is removed from saidjunction prior to said reheating step.
 15. A method of bonding twopieces of plastic material, taken from the group of crystallinethermoplastic materials, to one another to form an article, the methodessentially consisting of the steps of:heating at least one edge of eachof said two pieces to at least about the fusion temperature of saidcrystalline thermoplastic material; then butt joining said heated edgesto one another under pressure to thereby form a welded junction, saidpressure causing a bead of plastic material to be formed on at least oneside of said welded junction; and improving the physical and dielectricstrength of the material at the welded junction by subjecting the weldedjunction to a reheat treatment to cause the material to become molten,and quenching the molten material to reduce its temperature to atemperature at least below said fusion temperture.